The invention relates to a method for decreasing the sensitivity of optical waveguide fiber to hydrogen. In particular, the method markedly reduces hydrogen induced attenuation in single mode optical waveguide fiber in a wavelength band centered about 1530 nm.
Hydrogen can react with defects in silica based optical waveguide fibers to form unwanted signal absorption bands. A number of strategies have been developed to avoid the incorporation of hydrogen into the waveguide fiber, including sealed cables, hermetically coated waveguide fiber, and optical fiber cabling materials or coatings which act as hydrogen getters.
An example of the hydrogen getter approach is found in U.S. Pat. No. 5,596,668, DiGiovanni et al. (""668). The species for gettering or bonding with hydrogen, in this case a metal, is placed in the clad layer of the waveguide fiber. Diffusion of hydrogen into the light carrying portion of the waveguide is reduced and the waveguide is said to be hydrogen resistant. Care must be taken to prevent inclusion of the getter species into the core region and the part of the clad layer adjacent the core region. These regions carry the signal light and the presence of getter material in the regions would cause unacceptable signal attenuation. The ""668 patent at column 3, II. 65-67 and in FIGS. 2, 3, and 4 makes clear the getter material must be located away from the light carrying part of the waveguide. This limitation together with the fact that hydrogen diffusion is not completely eliminated makes this approach less than optimum.
Providing the waveguide with a hermetic coating does essentially eliminate hydrogen induced attenuation. However, the application of the coating involves an additional process step which adds considerable cost in terms of raw materials, equipment, and manufacturing rate. Extra measurement steps to insure the hermeticity of the coating are also required.
An alternative getter method is one in which the getter material is incorporated in the waveguide polymer coating or in the materials which make up the cable. Such alternatives involve additional expense and the materials must be such that they will not degrade or otherwise leave the host material for the life of the waveguide, which is usually estimated in decades.
U.S. Pat. No. 4,125, 388, Powers (""388 patent), discloses and claims a method for making high purity optical waveguides, especially waveguides having very low concentrations of water. The inclusion of water in the silica-based glass matrix gives rise to broad absorption bands in wavelength ranges otherwise well suited to signal transmission. The ""388 patent discloses and claims a method for making very low water waveguides by removing water from the soot preform during the step in which a soot preform is heated to fuse the soot particles into a glass. The ""388 patent discloses the use of Cl2 gas as a drying agent. The Cl2 may be fed directly to the preform or a metal halide gas, such as GeCl4 and SiCl4, may be used together with an oxidizing agent to produce Cl2 in the vicinity of the preform. The drying is carried out within a temperature range in which the soot will fuse into a dense glass.
In contrast to this drying method, the method of the present invention includes a step which precedes the drying step and which is carried out at a temperature below that at which the preform will be consolidated.
Thus there is a need in the waveguide fiber industry for a method of eliminating hydrogen sensitivity which method:
fits readily into the flow of the existing waveguide fiber manufacturing process;
does not cause a marked reduction in manufacturing rate;
is simple and cost effective; and,
is built into the glass itself and so is reliable over the life of the waveguide.
The novel method and the resulting waveguide fiber derived therefrom disclosed and described here, meet the need for a low cost hydrogen resistant waveguide which has excellent long term reliability and which overcomes the deficiencies in the art noted above.
One embodiment of the invention relates to a method of making a hydrogen resistant optical waveguide fiber. A soot preform is fabricated by any one of several methods known in the art such as outside vapor deposition or axial vapor deposition. The method can be extended to include a modified inside vapor deposition preform manufacturing method by lengthening the time between soot deposition and soot consolidation or by including an excess of GeCl4 or SiCl4 with regard to oxygen. By any of several methods known in the art, at least a part of the central core region of the soot preform is made to have a refractive index higher than at least a part of the surrounding cladding glass layer. These methods can include co-deposition of a soot in the central region to raise the refractive index, co-deposition of a soot in the surrounding layer to lower the index, or treatment of the soot of either region with index modifying gases such as fluorine. Thus, modification of the refractive index can be accomplished during soot deposition or after soot deposition but prior to soot consolidation.
In one preferred embodiment, the method of deposition is the outside vapor deposition process, and GeCl4 or SiCl4 are employed to deposit a GeO2 doped SiO2 core region onto a bait rod. This is preferably followed by deposition of at least a minimal amount of a SiO2 cladding region (additional cladding may also be deposited now or at a later stage, if desired). The bait rod is them removed, and the resultant soot preform can be treated in accordance with the invention. In one such embodiment, a metal halide gas (e.g. GeCl4) is flowed around the soot preform (and through the hole left by removal of the bait rod, if one was employed to make the preform). Note that in the novel method described herein, the metal halide gas is preferably in excess relative to oxygen. This is in contrast to the smaller metal halide to oxygen ratio which is advantageous in a drying process.
In one embodiment of the present novel method, the soot preform is heated to a temperature greater than about 800xc2x0 C. but less than the soot consolidation or sintering temperature. A metal halide gas which is a precursor of a glass forming metal oxide is then caused to flow through or about the hot, porous soot, preferably at a flow rate which is not less than about 0.2 standard cubic centimeters per minute (sccm) per 100 grams of soot glass. As is known in the art, the succeeding process steps can include sintering the soot to form a clear glass body, adding additional overcladding if needed or desired, and collapsing or sintering it, and then drawing a waveguide fiber from the resulting draw preform. A flow rate of about 1 sccm or more per 100 g of soot glass is preferred, although a flow rate as low as 0.2 sccm/100 g is effective to improve hydrogen resistance. There is essentially no process reason for placing an upper limit on the flow rate. Thus the upper limit is dictated by material cost and equipment capability. A rate of 1.0 sccm/100 g is well within the capability of the equipment used to dry and sinter the soot preform.
The action of the metal halide gas on the soot preform is typically substantially complete in 1 hour. Variability in soot density may require that soot preform be exposed to the metal halide gas for longer time periods or shorter time periods may be effective. A range of about 0.5 to 10 hours has been found to cover the normally encountered range of soot densities and temperatures. In a preferred embodiment of the method, the soot preform is held near in the range of about 1000xc2x0 C. to 1150xc2x0 C. during immersion in the metal halide gas flow. However, the method is effective at least to temperatures as high as 1250xc2x0 C.
The method works well when the index increasing core dopant is germania, although the method will be effective for other core glass dopants. Typical metal halide gases which may be used in the method include GeCl4 and SiCl4.
In an alternative embodiment, the same effect can be achieved by utilizing a soot deposition process, employing a metal halide precursor (GeCl4) during soot deposition, and employing less than a stoichiometric amount of oxygen in the reaction chamber. In this manner, incorporation of an adequate amount of reduced Ge can be supplied outside the GeO2 doped core.
A second aspect of the invention is a hydrogen resistant optical waveguide fiber made using the novel method.
A third aspect of the invention is a soot preform and a method of making a soot preform which is a precursor of a hydrogen resistant waveguide fiber. The method for making the soot preform includes the steps of depositing soot on any of several suitable soot collecting targets known in the art such as a bait rod of carbon, silica, or alumina, or on the inside or outside of a silica based glass tube. The soot comprises a silica layer and a core region of silica doped with an index raising material such as germania. Before sintering, the soot preform is heated and treated with a metal halide gas as before.
A fourth aspect of the invention is an optical waveguide fiber which contains reduced metal species (e.g. reduced germanium) in the core region or in the clad region immediately adjacent the core. The clad region immediately adjacent the core region is a ring of thickness 5 to 10 xcexcm surrounding the core region.
The presence of such reduced metal species is the result of the treatment of the soot preform with a metal halide gas. The reduced metal species may be detected and quantified by any of a number of methods in the art. For example, the presence of reduced Ge may be quantified by measuring the absorption by the waveguide or waveguide glass preform of light having a wavelength near 240 nm. Absorbance is equal to (1/t)log(Io/I), where t=sample thickness, Io=incident intensity, and I=transmitted intensity. In the case of a glass made from a germanium halide gas treated soot preform, it has been found that an absorbance of not less than about 0.3/mm of 240 nm light, at a radial point located outside the GeO2 doped region of the core, is indicative of hydrogen resistant glass. In one preferred embodiment, this region is present at or about halfway through the thickness of the adjacent clad ring, or greater than 1, more preferably greater than 3 microns outside the GeO2 doped SiO2 region. More preferably, the absorbance at this wavelength is less than about 0.2/mm. That is, sufficient reduced Ge is present in the glass to make a waveguide fiber which is hydrogen resistant.